1. Field of the Invention
The present invention relates to a novel acetylene compound, more specifically to a novel acetylene compound which is useful as a component for a liquid crystal composition used for a liquid crystal element, a liquid crystal composition containing said compound and a liquid crystal element using said liquid crystal composition.
2. Description of the Related Art
A liquid crystal display device is thin and light and is low in power consumption and therefore is used as a display in various uses.
A TN (twisted nematic) type display system is most widely used for a liquid crystal display element at present. However, this TN type display system has a response time as slow as 10 to 50 m second and therefore is inferior in terms of a response time as compared with a light-emitting type element (cathode tube, electroluminescence, plasma display and the like). Further, an STN (super twisted nematic) type display element in which an angle of twist is controlled to 180 to 270.degree. has been developed, but the response time is still inferior. Thus, while various efforts for the improvement have been made, a TN type liquid crystal display element having a shorter response time has not yet been achieved.
However, a novel display system using a smectic liquid crystal (ferroelectric liquid crystal) which is actively researched in recent years has a possibility of a marked improvement in a response time [N. A. Clark et al.; Applied Phys. Lett., 36, 899 (1980)].
This system is a method making use of a chiral smectic phase such as a chiral smectic C phase exhibiting ferroelectricity. It is known that not only the chiral smectic C phase but also chiral smectic F, G, H and I phases exhibit ferroelectricity. These smectic liquid crystal phases belong to tilted chiral smectic phases. Among them, a chiral smectic C phase which has a low viscosity and is expected to have a high speed response is studied to be used in terms of practical use.
Further, a display system making use of a chiral smectic CA phase (anti-ferroelectric phase) which is a higher-order phase of a chiral smectic C phase is actively researched in recent years [L. Chandani et al.; Jpn. J. Appl. Phys., 27L729 (1988)].
Further, in an anti-ferroelectric liquid crystal compound or liquid crystal composition, a smectic liquid crystal (TLAFLC) exhibiting a response of a V characteristic having no threshold value is found, and a display system making use of this is actively researched [S. Inui et al.; J. Mater. Chem., 6.71 (1996)].
Various liquid crystal compounds exhibiting these tilted chiral smectic phases have so far been studied, and a lot of compounds have already been found and produced. However, in order to actually use them as a high speed-response liquid crystal element, the following conditions have to be satisfied:
(a) a tilted chiral smectic phase is exhibited in a wide temperature range including room temperature, PA1 (b) the viscosity is low, PA1 (c) a suitable tilt angle is found PA1 (d) the spontaneous polarization is large to some extent, and PA1 (e) the aligning property is good.
However, no single compound which can satisfy these conditions has ever been obtained.
Accordingly, a liquid crystal compound exhibiting a tilted chiral smectic phase is used as a liquid crystal composition exhibiting a tilted chiral smectic phase by mixing with other several liquid crystal compounds exhibiting a tilted chiral smectic phase or a tilt base smectic phase. In any event, however, the liquid crystal compound exhibiting a tilted chiral smectic phase has to have a wide mesomorphic temperature range, a small viscosity and a spontaneous polarization which is large to some extent.
For example, the following compound is known as a liquid crystal compound exhibiting a tilted chiral smectic phase [Japanese Patent Application Laid-Open No. Sho 59-219251 (U.S. Pat. Nos. 4,596,667 and 4,820,444)]: ##STR1##
wherein m is 0, 1 or 2; R* represents an optically active alkyl group; Ring A represents a 1,4-phenylene group or a 1,4-trans-cyclohexylene group; and X represents a linear or branched alkyl group or alkoxy group each of which has 1 to 18 carbon atoms.
However, the present inventors investigated the above compound to find the problems that while a compound in which X was an alkoxy group had a high upper limit temperature of the chiral smectic phase (the chiral smectic C phase), it had a high viscosity and that a compound in which X was an alkyl group had a low viscosity but had a low upper limit temperature of the chiral smectic phase.
Further, the following compound [Japanese Patent Application Laid-Open No. Sho 60-32748 (U.S. Pat. No. 4,576,732) and Japanese Patent Application Laid-Open No. Hei 3-83951 (U.S. Pat. No. 5,116,531)] had the problem that when the long chain portion (R in the following formula) at the reverse side to the optically active group was an alkoxy group, the upper limit temperature was high and the viscosity was high as well, and that when it was an alkyl group, the viscosity was low and the upper limit temperature was low as well: ##STR2##
wherein R represents an alkyl group or alkoxy group each of which has 20 or less carbon atoms; R* represents an optically active alkyl group and contains an asymmetric carbon atom; and m and n are natural numbers, provided that when m and n are 1, R has 7 or more carbon atoms.
Further, in the case of a compound having the following skeleton (for example, Japanese Patent Application Laid-Open No. Hei 4-103560), the present inventors compared a compound in which a terminal group represented by R shown below was an alkyl group with a compound in which it was an alkoxy group to find that there was the problem that the compound having the alkoxy group had a high upper limit temperature but had a high viscosity, and the compound having the alkyl group had a low viscosity but had a low upper limit temperature: ##STR3##
As described above, in the compounds which have so far been known, the terminal groups which are not optically active are bonded through an --O-- bond (alkoxy) or a single bond (alkyl) in many cases, and in the case of these compounds, it has been impossible that they have both of a high upper limit temperature and a low viscosity.
Further, a compound into which an acetylene bond is introduced as a bonding group is known (Japanese Patent Application Laid-Open No. Hei 8-81417). However, referred to in the specification of the above compound is nothing about compounds in which a linear terminal group is linked through an acetylene bond, and only the compounds in which a group having an asymmetric carbon atom is bonded through an acetylene bond are described. Further, in the case of the compound into which an optically active group is introduced via an acetylene bond, the acetylene bond is present between the optically active portion and the central skeleton of the molecule of the liquid crystal compound, and therefore there has been the problem that it is impossible to introduce a large dipole between them, so that the compound exhibiting a large spontaneous polarization can not be obtained.
As described above, conventional liquid crystal compounds and liquid crystal compositions exhibiting a tilt base chiral smectic phase are still unsatisfactory in terms of physical properties such as a mesomorphic temperature range, a viscosity and a spontaneous polarization and are expected to be improved therein.